1. Field of the Invention
This invention relates to an analog-to-digital converter (hereinafter referred to as an ADC), and more particularly to an integrated ADC as used in the field of welfare electric equipments, home electric apparatus, etc.
2. Description of the Prior Art
In a prior art, an ADC comprises an input terminal to which an analog input signal is applied, a capacitor connected to the input terminal through a switch, for storing the analog input signal, a discharging circuit for discharging the charge stored in the capacitor, a level detection circuit connected to the input terminal of the capacitor and a counter connected to the output terminal of the level detection circuit for counting the number of clock pulses. An example of the prior ADC is shown in G. Smarandoin, et al. "An All MOS Analog to Digital Converter Using a Constant Slope Approach", IEEE Journal of Solid-State Circuits, June 1976.
In this ADC, however, if an input signal voltage is lower than the detection level voltage V.sub.TH, the discharge time T cannot be detected, so that an analog to digital conversion (called A-D conversion hereinafter) is not performed.
In order to perform the A-D conversion over a wide range of the input signal, some of the inventors of this invention previously developed a new type ADC, for which a patent application was filed as U.S. patent application Ser. No. 887,966 on Mar. 20, 1978. The above new type ADC has a bias voltage supply means connected to the capacitive element, which shifts the potential at the input terminal of the capacitor during the discharge time T.
But, in this circuit, when the bias voltage V.sub.B is larger than the detection level voltage V.sub.TH and the potential of the output terminal of the capacitor changes from the bias voltage V.sub.B to the ground voltage, a negative voltage (generally reverse polarity of the input signal one) is produced on the input terminal of the capacitor. Therefore, if the new type ADC is constructed with n-channel MOSIC, the circuit may be broken or may have an extraordinary action.